Microscopic examination of the pituitary reveals two distinct types of tissue: epithelial and neural (
Fig. 41.2 ). The epithelial portion of the human pituitary gland is called the adenohypophysis. The adenohypophysis makes up the anterior portion of the pituitary and is often referred to as the anterior lobe of the pituitary, and its hormones are referred to as anterior pituitary hormones. The adenohypophysis is composed of three parts: (1) the pars distalis, which makes up about 90% of the adenohypophysis, (2) the pars tuberalis, which wraps around the stalk, and (3) the pars intermedia, which regresses and is absent in adult humans.
The neural portion of the pituitary is called the neurohypophysis and represents a downgrowth of the hypothalamus. The most inferior portion of the neurohypophysis is called the pars nervosa, also called the posterior lobe of the pituitary (or simply posterior pituitary). At the superior end of the neurohypophysis, a funnel-shaped swelling called the median eminence develops. The portion of the neurohypophysis that extends from the median eminence down to the pars nervosa is called the infundibulum. The infundibulum and the pars tuberalis make up the pituitary stalk—a physical connection between the hypothalamus and pituitary gland (see
Fig. 41.2).
The pituitary gland (anterior and posterior lobes) is situated within a depression of the sphenoid bone called the sella turcica. Generally, cancers of the pituitary have only one way to expand, which is up into the brain and against the optic chiasma. Thus any increase in size of the pituitary is commonly associated with visual field or visual acuity abnormalities and headaches. The sella turcica is sealed off from the brain by a membrane called the diaphragma sellae.
The Neurohypophysis
The pars nervosa is a neurovascular structure that is the site of neurohormone release adjacent to a rich capillary bed. The peptide hormones that are released are antidiuretic hormone (ADH, or arginine vasopressin) and oxytocin. The cell bodies of the neurons that project to the pars nervosa are located in the supraoptic nuclei (SON) and paraventricular nuclei (PVN) of the hypothalamus (a nucleus refers to a collection of neuronal cell bodies residing within the central nervous system (CNS); a ganglion is a collection of neuronal cell bodies residing outside the CNS). The large cell bodies of these neurons are described as magnocellular, and they project axons down the infundibular stalk as the hypothalamohypophyseal tracts. Individual magnocellular neurons are hormone specific, producing •Fig. 41.1 Cross-sectionalimageoftheheaddemonstratingtheproximityofthehypothalamusandpituitaryglandandtheirconnectionbyaneurohypophyseal(pituitary)stalk.
either ADH or oxytocin. These axons terminate in the pars nervosa (
Fig. 41.3 ). In addition to axonal processes and termini from the SON and PVN, there are glial-like support cells called pituicytes. The posterior pituitary is extensively vascularized and the capillaries are fenestrated, thereby facilitating diffusion of hormones into the systemic circulation.
Synthesis of ADH and Oxytocin
ADH and oxytocin are small peptides (nine amino acids) that differ in only two amino acids, yet they have limited overlapping activity. ADH and oxytocin are synthesized as preprohormones (